The pathogenesis of Parkinson disease (PD) is unknown but dopamine-induced oxidative stress, proteasomal abnormalities and mitochondrial dysfunction are associated with its neurodegeneration. Rare heritable forms of PD are linked to an increasing number of gene loci. At the PARK1 locus, SNCA encodes a neuronal protein, alpha-synuclein (alpha-S), that is involved in the transition of synaptic vesicles from the reserve-resting pool to the readily releasable pool in vivo and in vitro. It is linked to sporadic PD by the formation of fibrillar inclusions that contain phosphorylated alpha-S, and to autosomal dominant PD by a likely gain-of-function effect of two infrequent point mutations. The PARK2 gene encodes parkin, an E3 ubiquitin ligase. It is mutated in <50% of all autosomal recessive PD cases by a probable loss-of-function phenomenon. In normal human brain (but not rat brain), a pool of alpha-S undergoes O-linked glycosylation, thereby generating alpha-Sp22. This glycoprotein is a substrate for parkin's E3 ligase function in vitro and accumulates in PARK2-mutant PD brain. The central hypotheses of this application state that 1) a shared pathogenetic pathway is encoded by PD-linked genes, 2) characterization of the alpha-S glycosylation in primate brain will provide insights into the pathogenesis of PD, 3) the normal function of the Parkin E3 complex is essential for the sustained survival of catecholaminergic neurons in adult human brain, and 4) the identification of the in vivo subunits of the assembled parkin E3 complex will validate reported binding partners and reveal potentially neurotoxic substrates. To this end, I have identified two Specific Aims: Aim 1: To characterize the glycosylation of alpha-S in human control brain as well as PARK1-linked PD brain and to model its biosynthesis in a cell model, and Aim 2: To biochemically purify the subunits of the Parkin E3 ligase complex from human brain, and verify them in vitro.